1. Technical Field
This invention relates to a power control circuit which is particularly, though not exclusively, suited to the ballasting of low and high pressure sodium, mercury arc and metal halide discharge lamps (high intensity discharge lamps or HID lamps). Typically such systems can be used for highway lighting, architectural floodlighting, warehouse and industrial lighting etc.
2. Background of the Invention
Traditionally, ballasting for HID lamps is by use of inductors or chokes capable of controlling the lamp current through the impedance they present in series with the mains supply voltage. With some types of HID lamp a high striking voltage, typically 4-5 kV, is required to ionize the gas filling the tube and initiate the arc.
In prior art systems for ballasting HID lamps, the lamp ballasting means and the lamp striking means are typically discrete circuit elements. Historically, HID lamps have been ballasted by using the impedance of a series connected inductor for controlling the lamp current and a separate starter or igniter module to provide the necessary high voltage to strike the lamp. FIG. 1 illustrates a typical arrangement for prior art electronic ballasts for HID lamps. A conventional power factor controller is formed by transistor TR1, inductor L1, diode D1 and capacitor C1. Alternating positive and negative output voltage is provided to the lamp by a full bridge arrangement comprising four transistors TR3, TR4, TR5, TR6. The transistors are alternately switched on and off in complementary pairs TR3, TR6 and TR4, TR5 at a low frequency, typically 100-200 Hz.
Connected in series with the lamp across the bridge is an igniter circuit comprising pulse transformer TX1, a Sidac, capacitor C3 and resistor R1. When the igniter circuit operates, the capacitor C3 charges through resistor R1 to a voltage at which the Sidac device switches on, discharging the capacitor C3 into the primary winding of the transformer TX1. The voltage applied to the transformer primary is multiplied by the high turns ratio of the transformer and is sufficient to ionize the gas filling the lamps arc tube, thereby initiating an arc.
Since the voltage is AC, the arc will be extinguished when the lamp current approaches zero and the voltage applied to the tube is subsequently reversed. Therefore the igniter must operate again in the opposite voltage half cycle to re-strike the arc for the flow of current in the opposite direction. This ignition cycle is repeated until the lamp electrodes are sufficiently heated by the arc current for thermionic emission to take place. Then the arc voltage in the tube falls below the threshold voltage of the Sidac and arc current is maintained without operation of the igniter circuit.
A further transistor TR2 controls the flow of current in the output bridge circuit and consequently controls the lamp current. Transistor TR2 is turned on until the current in inductor L2 reaches a preset threshold value, then the transistor TR2 is turned off. Current continues to flow via a diode D2 until the current has decayed to another preset threshold value, then the transistor TR2 is turned on again.
Because of the high rate of rise of voltage, the capacitance and inductance of the wiring to the lamp act to attenuate the high voltage ignition pulse to the lamp, so limiting the practical length of the wiring between the igniter circuit and the lamp.
In recent years it has become known to use high frequency ( greater than 20 kHz) electronic ballasts to supply lamp current for fluorescent lamp installations, giving longer tube life due to lower tube current crest factor and higher overall efficiencies due to reduced power losses in the ballast and tube. Attempts have been made to design high frequency electronic ballasts for HID lamps with some level of success but these are fraught with problems due mainly to the predisposition of many HID lamps to acoustic arc resonance when operated at frequencies substantially above line frequency. It is in some cases possible to design high frequency ballasts specifically for one type and size of HID tube if the operating frequency of the ballast is chosen carefully but if the gas pressure in the tube changes substantially during the life of the tube the resonance characteristics will also change and could cause catastrophic failure of the arc tube. This will be at best alarming and could be dangerous if the arc tube fragments are not sufficiently contained within the lamp fitting (luminaire).
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and system for avoiding acoustic arc resonance (AAR) during steady state operation of a high intensity discharge lamp. A lamp ballast generates a low frequency current with a high frequency ripple current. The high frequency ripple current is frequency modulated. Lamps with different characteristics may be operated by the same type of ballast using fewer switching components.